(a) Field of the Invention
The present invention relates to a non-magnetic mono-component toner composition, used for non-magnetic mono-component developing system in which a developing roller and a photoreceptor contact each other, and a preparation method thereof.
(b) Description of the Related Art
In general, dry developing method in electrophotography can be largely classified into dual-component developing system using a dual-component developer comprising a toner and a carrier, and mono-component developing system using a mono-component developer comprising a toner only. Between the two, the mono-component developing system is advantageous in compactness, cost, and maintenance. The mono-component developing system is further classified depending on whether a magnetic toner or a non-magnetic toner is used. The system of using non-magnetic toner is advantageous in that color printing is possible. Accordingly, copiers and printers employing non-magnetic mono-component developing system are coming into wide use, and the printing speed is improving significantly.
Differing from the dual-component toner using a dual-component developer comprising carrier particles that help the transfer of toner particles or the magnetic toner using magnetic force to transfer toner particles, the fluidity of toner particles themselves greatly affects the transfer characteristics in the non-magnetic mono-component toner. That is to say, fluidity is the most important physical property of a non-magnetic mono-component toner. It greatly affects copying and printing image quality, image density, fogging (image contamination by toner transfer from the non-imaging region), cleaning characteristics of the toner, etc. If the toner particles have insufficient fluidity, stripped line contamination arises in the copied or printed image. This is because the non-magnetic mono-component toner stagnates at the region between the developing roller and the sub-roller for providing the toner to the developing roller, and thus it is not smoothly transferred to the surface of the developing roller.
In addition to fluidity and toner blocking, the toner characteristic related with charging mechanism of the photoreceptor is also an important property. In general, charging mechanism of copiers or printers is classified into the type in which the charger does not contact the photoreceptor (e.g., corona charging mechanism), and the method in which the charger contacts the photoreceptor (e.g., roller charging mechanism). Of the two, the roller charging method is widely adopted because it generates less ozone. In the roller charging method, the primary charge roller (PCR) is contacted with the photoreceptor to charge the surface of the photoreceptor. If there is contamination on the surface of the PCR because of toner attached to it, image contamination such as fogging and uneven image takes place. In particular, because the fluidity decreases further at a low temperature, the toner tends to attach strongly to the surface of the PCR, thereby impairing charging characteristics and transfer efficiency.
Easy removing fine particles such as silica have been included in the toner particles in order to improve fluidity and charging ability of the toner at a low temperature. The fine particles reduce adhesion force of the toner to the drum, and thereby improve the transfer efficiency. To attain good transfer efficiency, many fine particles have to be coated on the toner surface, but this may cause lowering of charging ability of the toner, adhesion of the fine particles to the carrier causing an electrostatic latent image, etc., filming, fixing problems, and so forth. In particular, because silica particles are greatly environment-dependent, uneven imageuneven image may occur at a low temperature and humidity, and contamination at the non-imaging region may occur at high temperature and humidity.
To solve the environment-dependency of toner charging, inorganic fine particles such as titanium oxide, which have lower electrical resistance and better charge exchangeability compared with the silica particles, have been added. However, charge distribution of the toner may change easily in this case.
To solve this problem, a method of increasing electrical resistance by treating the surface of the inorganic fine particles with a silane couping agent, etc., has been proposed. In this case, the fine particles have a stronger coagulation property, so that dispersibility on the toner surface is reduced and the charge exchangeability is decreased, which may cause a decrease of toner fluidity or blocking by free coagulated particles.
Also, differing from the magnetic mono-component toner, in which the toner is easily transferred to the developing roller by magnetic attraction, the thickness of the toner layer is controlled by pressing a blade made of metal or rubber on the developing roller and the toner is charged, when using the non-magnetic mono-component toner. Accordingly, because a pressure is applied on the non-magnetic mono-component toner, the toner may block the developing roller and the blade if used for a long time, which may make the thickness of the toner layer on the developing roller and the degree of charging non-uniform, thereby causing fogging or shading.
In order to prevent toner blocking, the glass transition temperature (Tg) and molecular weight of the binder resin comprised in toner mother particles have been increased. However, this inevitably worsens the fusing property of the toner and the flatness of fused images, and makes the image non-uniform. Although this method is advantageous in preventing toner blocking on the developing roller at a high temperature, the low temperature double image problem at the non-imaging region because of failure to remove the toner remaining on the photoreceptor surface, arises at a low temperature, especially in the non-magnetic mono-component development system in which the cleaning process is omitted.
Thus, with regard to non-magnetic mono-component development system, in which the developing roller contacts the photoreceptor and the cleaning process is omitted, a non-magnetic mono-component toner resistant to changes in the environment and capable of solving the low temperature double image problem is necessary.